In an offshore installation where a conventional jacket is used to support a well, the jacket itself rests on the ocean bottom. In order to keep the jacket from sinking into the ocean bottom, it is common to affix a plurality of large, flat, rigid plates, known as mudmats, to the bottom of the jacket. The mudmats rest on the sea floor and support the weight of the jacket until the piles are installed. The mudmats are designed to temporarily distribute the weight of the jacket and prevent it from sinking into the floor until the installation of the piles and the connection of the piles to the pile sleeves of the conventional jacket is completed. Once the piles have been installed and connected to the jacket, the weight of the jacket is carried by the piles and the mudmats become redundant.
The size of the mudmats required depends on the soil conditions at the ocean floor. In instances where the ocean floor is soft, the mudmats required are enormous in size and weight. Furthermore, in instances where the soil is extremely soft, besides large mudmats, a lift vessel is required to hold the conventional jacket so that it does not sink and remains level while piles are being installed. In addition, the pile installation procedure requires its own lift vessel. Therefore, two lift vessels may be required for pile installation. The first holds the jacket while the piles are being installed and connected to the pile sleeves of the jacket, and the second performs the pile installation.
Because the cost of providing any piece of equipment offshore is relatively high, it is desired to provide a technique for installing a jacket using a caisson without requiring either the extended use of jackup construction platform or large mudmats that are currently required.
In another scenario, a single well may have already been drilled by driving a caisson into the ocean floor. As disclosed in U.S. Pat. No. 5,012,875, a structural frame can be installed and connected to the caisson. The frame includes a plurality of pile sleeves, through which piles are driven into the ocean floor. The objective is to laterally brace the caisson and use the caisson to support a deck in order to support equipment needed to drill and produce a well. In other configurations, disclosed in U.S. Pat. Nos. 4,842,446 and 5,051,036, both the piles and the caisson support the deck on the top. In each of these inventions, the idea is to strengthen the caisson so that it can be used to support the deck. The caisson is pre-existing in all these cases and is attached to the side of the structural frame.
In these other configurations, the structural frame is attached to one side of the caisson, before the piles are installed, the weight of the frame tends to bend the caisson somewhat. It is usually necessary to provide a lift vessel to support the opposite side of the frame so as to maintain it level until the piles have been installed. Hence, it is desirable to provide a system for supporting a jacket during pile installation without incurring the expense of multiple lift vessels or extended lift vessel usage.